The human precorneal tear film is comprised of three primary layers, each of which serves a specific function. The innermost layer of the precorneal tear film provides a protective environment for the superficial epithelial cells of the cornea and helps protect against microbes and foreign bodies. The outer surface of the precorneal tear film is the primary refracting surface of the eye. Its surface tension helps to smooth this surface, thus improving the optical quality of the image ultimately impacting the retina. Additionally, the precorneal tear film provides a lubricating function during blinking. These structures are often disrupted in dry eye conditions, which are some of the most common ophthalmic disorders seen by eye-care practitioners. Dry eye disorders and/or disease can lead to premature breakup of the tear film after a blink, leading to damage of the superficial epithelium which may result in discomfort and be manifested as optical blur. In addition, the ability of a patient to wear contact lenses is a direct function of the quality and quantity of the tear film, and dry eye disorders and/or disease therefore has a significant impact on contact lens wear parameters.
The precorneal tear film is comprised of an inner mucin layer, a middle aqueous layer, and an outermost thin lipid layer. Various treatments are used in an attempt to alleviate dry eye symptoms. For example, it has been proposed to treat certain dry eye conditions by the application of heat and pressure to unclog meibomian glands, or with pharmaceutical methods to unclog meibomian gland and to stimulate tear production.
Notwithstanding the foregoing, it has been a long standing and vexing problem for clinicians and scientists to objectively demonstrate an improvement in the precorneal tear film thickness at the conclusion of the proposed treatment. Further, many promising treatments for dry eye have failed to receive approval from the United States Food and Drug Administration due to the inability to demonstrate clinical effectiveness to the satisfaction of the agency.
In response to the foregoing long felt need, various methods of measuring the thickness of the precorneal tear film, and specifically the lipid layer thereof have been proposed. For example, Korb, one of the inventors of this invention provided an overview and background of his invention of a specular reflection microscope system that allowed quantification of the tear film lipid layer thickness based on the interference colors of the lipid layer. This system included a hemi-cylindrical broad spectrum illumination source with heat absorbing filters, a binocular microscope with a Zeiss beam-splitter providing 70% light to a high resolution video camera, a VHS recorder, and a high resolution 20-inch color monitor. Following calibration with Eastman Kodak color reference standards (Wratten filters), the static and dynamic appearance of the lipid layer was observed before and after blinking. During the observation period, the subject was instructed to blink naturally while gazing at a fixation target. For purposes of quantization and standardization, a specific region of the tear film was designated for analysis. This area encompassed a zone approximately one mm above the lower meniscus to slightly below the inferior pupillary margin, averaging 7-8 mm wide and 2.5 mm in height. The dominant color of the specularly reflected light within this designated area was used as the basis for assigning lipid layer thickness values. Thickness values were assigned to specific colors on the basis of prior work on tear film lipid layer interference colors (McDonald, 1969; Nom.; 1979; Guilon, 1982; Hamano et al., 1982) and are summarized in Table 1. To confirm the lipid layer thickness values assigned to each subject's tear film lipid layer, recordings were independently graded by two observers masked as to subject identity. (Korb, D R, Baron D F, Herman J P, et al., Tear Film Lipid Layer Thickness as a Function of Blinking, Cornea 1994:13:354-9). While the foregoing apparatus was effective in measuring improved lipid layer thickness, measurement inaccuracies were nevertheless introduced into the system. Working backwards, the color monitor had to be provided with a sufficient input signal to enable the lipid layer to be imaged on to the monitor screen. The foregoing thus required a minimum illumination to be provided to the slit lamp, of which 70% was directed to the high resolution video camera. This, in turn, dictated the minimum amount of light required to illuminate the corneal surface. Thus, the amount of light required to make the foregoing system operational was not optimum as it interfered with the naturally occurring tear film as the heat generated by the light caused tear film evaporation. Further, the amount of light required to make the system functional caused some degree of reflex tearing.
Another apparatus for measuring the tear film is disclosed in European Patent Application EP 0 943 288 assigned to Kowa Company, Ltd. of Japan. The application discloses an apparatus for the non-contact measurement of the quantity of lacrimal fluid collected on the lower eyelid. According to the invention, tear volume is calculated from a measurement of the volume of fluid pooled at the lid eye meniscus. While knowledge of the total volume of fluid may be of some use to eye-care practitioners, it does not specifically measure the lipid layer thickness or its improvement as the result of a particular treatment regimen.
U.S. Pat. No. 4,747,683 to Marshall G. Doane discloses a Method and Device for in Vivo Wetting Determinations wherein a contact lens is illuminated with coherent light and the pre-lens tear film is imaged in such a way as to form an interference pattern. The image formed thereby is recorded and the tear film thickness is determined by correlating the interference bands of the recorded image. A coherent light source and a camera are focused at the pre-lens film to image specularly reflected light from the front and rear surfaces of the tear film. A film motion analyzer provides numerical coordinates of interference bands, and a microprocessor analyses the coordinates to provide a quantitative measure of lens position or wetting characteristics. Again, while knowledge of the tear film thickness covering the contact lens surface may be useful in the context of contact lens fitting, the Doane apparatus does not specifically measure lipid layer thickness on the natural eye.
Another instrument that purports to measure tear film lipid layer thickness is the Tearscope Plus manufactured by Keeler Instruments Inc., of Broomall, Pa. and Berkshire, UK. More specifically, the Tearscope is a hand-held or slit lamp mounted device that comprises a tubular housing which contains a coaxially mounted cylindrical light source. The interior bore of the housing is covered with a cylindrical diffuser plate that diffuses the light. In use, the eye-care practitioner places one end of the tube proximate the patient's eye thus illuminating the whole eye, including the pupil, and observes the interference patterns on the pupil surface through the opposite end of the tube. The color of the interference pattern generated by blinking is then correlated to tear film thickness. The Tearscope is not without its inherent drawbacks and deficiencies as the process by which the eye is illuminated and the measurement is made introduces error which is diagnostically unacceptable. For example, the proximity of the illuminator to the eye surface when combined with the light intensity required to obtain a viewable interference pattern can cause reflex tearing. In addition, the illumination system employed illuminates the entire eye, including the pupil. Thus, light from the Tearscope is directed on to the retinal surface which, in turn causes a proprioceptive response which also skews measurement accuracy.
In view of the foregoing, it is an object of the present invention to provide a method and apparatus that overcomes the drawbacks and deficiencies of the prior art.
Another object of the present invention is to provide a method and apparatus that allows the accurate measurement of the thickness of the lipid layer component of the precorneal tear film.
A further object of the present invention is to provide a method and apparatus wherein the lipid layer thickness of the precorneal tear film may be measured without the introduction of reflex tearing.
A still further object of the present invention is to provide a method and apparatus that enhances contract and thereby the observability and measurablity of the lipid layer thickness of the precorneal tear film.
Yet another object of the present invention is to provide a method and apparatus for measuring the lipid layer thickness of the precorneal tear film using a low level of light in order to minimize tear film evaporation that can alter the measurement.
Another object of the present invention is to provide a method and apparatus for measuring the lipid layer thickness of the precorneal tear film wherein the patient is comfortable during the examination.
Another object of the present invention is to provide a method and apparatus for measuring the lipid layer thickness of the precorneal tear film that minimizes light entering the pupil to minimize reflex tearing and proprioceptive responses that can alter the measurement.